Optimize asyncio.Future schedule/add/remove callback

[1st1]

BPO	32348
Nosy	@asvetlov, @1st1
PRs	#4907
Files	t.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/1st1'
closed_at = <Date 2017-12-18.01:22:51.716>
created_at = <Date 2017-12-16.23:50:13.245>
labels = ['3.7', 'expert-asyncio']
title = 'Optimize asyncio.Future schedule/add/remove callback'
updated_at = <Date 2017-12-18.01:22:51.715>
user = 'https://github.com/1st1'

bugs.python.org fields:

activity = <Date 2017-12-18.01:22:51.715>
actor = 'yselivanov'
assignee = 'yselivanov'
closed = True
closed_date = <Date 2017-12-18.01:22:51.716>
closer = 'yselivanov'
components = ['asyncio']
creation = <Date 2017-12-16.23:50:13.245>
creator = 'yselivanov'
dependencies = []
files = ['47335']
hgrepos = []
issue_num = 32348
keywords = ['patch']
message_count = 3.0
messages = ['308481', '308502', '308503']
nosy_count = 2.0
nosy_names = ['asvetlov', 'yselivanov']
pr_nums = ['4907']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue32348'
versions = ['Python 3.7']

[1st1]

Key observation: 99.9% of the time Futures and Tasks have only one callback.

Currently we have a list of callbacks in each Future/Task. We can avoid list object allocation if we add a field to store the first callback. This way we'll only need to ever allocate the list if a Future/Task has more than one done callbacks.

asyncio with the proposed patch applied shows that 3-6% better performance of an asyncio echo server implemented using asyncio.streams. This benchmark involves quite a bit of other Python code, so this improvement is actually quite significant.

The patch consists of:

1. first callback / callbacks list refactoring.

2. a free list implementation for Future.__await__ objects (which are created on almost all awaits in asyncio).

3. a big cleanup of the code, ensuring that Future/Task are always initialized properly.

[1st1]

New changeset 1b7c11f by Yury Selivanov in branch 'master':
bpo-32348: Optimize asyncio.Future schedule/add/remove callback. (bpo-4907)
1b7c11f

[1st1]

The attached benchmark from https://bugs.python.org/issue32204 shows that the updated Task is 15% faster.